Stainless steel, especially austenitic stainless steel, has been widely employed for its superior corrosion resistance property and its capability of being decorated. Particularly, fasteners such as a bolt, a nut, a screw, a washer and a pin are made of austenitic stainless steel in view of these properties. However, strength itself of the above austenitic stainless steel products differs from that of carbon steel so that the strength of the above products is improved mostly in an intermediate processing step before a final step to make each figure thereof. For example, crystal structure of the austenitic stainless steel is closely tightened by press working, extrusion molding, panting and the like so as to strengthen the material itself. Such improvement of the strength in the intermediate processing step is necessarily limited because there are restrictions to shape the material into a specific figure by the figure such as a bolt or a nut and also to lower cost of a mold in the extrusion molding and the like. Therefore, when higher strength, anti-seizure, a tapping capacity on a steel plate are demanded on austenitic stainless steel products such as a bolt, a nut and a screw, the following methods are available. 1 Hard chrome plating or wet type metal plating such as Ni--P, 2 coating such as physical vapor deposition, abbreviated to PVP hereinafter, or 3 hardening treatment by penetration such as nitriding or the like.
However, the above methods such as the wet type metal plating or the coating like PVD have drawbacks of shortening product lifetime due to peeling of a coat formed on the surface of the austenitic stainless steel products and the like.
Further, the above nitriding comprises penetrating nitrogen atoms from the surface of austenitic stainless steel inside thereof so as to form the surface lawyer into a hard nitrided one. In this method, the surface hardness of austenitic stainless steel products is improved, however, a vital problem of deteriorating an essential property of anti-corrosion is caused. Furthermore, there are other drawbacks that the surface roughness of the products deteriorates, the surface blisters or the products are magnetized. It is thought that nitriding deteriorates anti-corrosion property because chrome atoms (which improve anti-corrosion property) contained in the austenitic stainless steel are consumed as chrome nitrides such as CrN and Cr.sub.3 N by nitriding and their content lowers. Still further, there are problems that the surface blisters, the surface roughness deteriorates or the like.
As the other methods for the above penetration treatment for hardening, there is carburizing. However, a conventional carburizing method comprises contacting the surface of austenitic stainless steel products with a gas containing carbon so as to invade the carbon atoms into the surface layer and to form a hard carburized layer. In this method, carburizing is generally conducted at a temperature not less than 700.degree. C. of an A1transformation temperature of iron by considering the permeability of carbon atoms and a limit of solid solution. This means that the austenitic stainless steel products have been maintained at a temperature far beyond the recrystallization (N.B. a temperature of recrystallization of iron is about 450.degree. C.) for a long time, resulting in remarkable deterioration of the strength, which is a great drawback. Since this carburizing method has the drawback that the material strength itself deteriorates greatly, its application to austenitic stainless steel products, which do not have originally so much hardness, is not being taken into consideration. In addition, it is true that an improvement of strength on fasteners such as a bolt, a nut or a screw is realized by press working, extrusion molding or panting as mentioned above to improve the entire hardness, so that an application of a technique to improve only the surface by carburizing is not considered.